As an example of a solvent to be used in the solvent extraction-infrared absorption method, carbon tetrachloride (hereinafter, this solvent will be shown as CCl.sub.4) is a well known substance. However, regarding the use of CCl.sub.4, the following two points should be noted. That is, its threshold limited value (hereinafter referred to as TLV) to the environmental atmosphere where it is practically used is only 10 ppm and its lethal dose (defined as the amount of poisonous substance which kills 50% of test animals when administered orally (hereinafter referred to as LD.sub.50)) is 9.6 g/kg. Therefore, the use of CCl.sub.4 in a limited space such as in a ship is especially undesirable.
With such a background, the availability of 1,1,2-trichloro-1,2,2-trifluoroethane (CCl.sub.2 F-CClF.sub.2, hereinafter called Flon-113) belonging to a series of fluorocarbon compounds has been recognized as a good solvent to be used in place of CCl.sub.4. That is, since the poisonous property of Flon-113 is so weak, its TLV being 1000 ppm and LD.sub.50 being 40 g/kg of a mouse's body, it is used as an important solvent in dry cleaning. Also, it is very often used as a solvent for the deoiling of a metal surface. Moreover, it is reported that Flon-113 is an effective solvent for use in the extraction of an oil fraction in water, since it has a large ability to dissolve oily substances in water, almost equal to that of CCl.sub.4. Furthermore, it has a characteristic behavior such that it does not absorb any infrared spectrum in the neighborhood of 2900 cm.sup.-1, which corresponds to a characteristic infrared absorption due to the existence of hydrocarbons (in the present case, the main skeleton of oil fractions in water can be considered to be hydrocarbon).
However, on the other side, even if Flon-113 is taken into consideration as a solvent to be used in the apparatus based upon the solvent extraction-infrared absorption method, its low boiling point (+47.6.degree. C.) is a hindrance to its use for such a purpose. In the temperature range from 0.degree. to 50.degree. C. which is the probable air temperature at which the measuring apparatus is operated, Flon-113 has a large vapor pressure. Accordingly, it is fugacious. For example, at a temperature higher than 30.degree. C., the vapor pressure of Flon-113 is more than three times of that of CCl.sub.4 and this large vapor pressure will surely bring about an inevitable escape of Flon-113. Moreover, the interface between the solvent and the aqueous layer becomes indistinct, resulting in an ineffective separation of the two layers. Furthermore, since the temperature of a Flon-113 solution in a cell increases to near its boiling point during a measurement by the absorption of infrared spectra, this will again enlarge the error in the determination of oil content. Therefore, Flon-113 can never be practically used as a solvent in such apparatus.
After such an experimental confirmation as mentioned above, the present inventors have made a series of experimental tests on other chlorinated hydrocarbons such as 1,1,2,2-tetrachloro-1,2-difloroethane (CCl.sub.2 F-CCl.sub.2 F, hereinafter called Flon-112) and tetrachloroethylene, which have a higher boiling point than Flon-113. However, the conclusion was that both compounds cannot be used as the solvent for the present purpose based upon the reason that the freezing point of the former is too high (+26.degree. C.) and it is in a solid state at ordinary temperatures. Further, the latter compound contains hydrocarbon skeletons originating from the existence of phenol resins and propylene oxide which are ordinarily added in order to increase its chemical stability.